Solve common problems in your JavaScript code

코드가 작동하지 않거나 브라우저가 무언가가 undefined라고 불평하면 모든 변수 이름, 함수 이름 등을 정확하게 입력했는지 확인하십시오.

문제를 일으키는 몇 가지 일반적인 내장 브라우저 함수는 다음과 같습니다.

세미콜론의 위치가 잘못 되지 않았는지 확인하세요. 예를들어:

함수와 관련하여 실수할 수 있는 것들이 많이 있습니다.

가장 흔한 실수는 함수를 선언하고 어디서도 호출하지 않는 것입니다. 예를 들어:

function myFunction() {
  alert("This is my function.");
}

위 코드는 호출하지 않는 이상 아무것도 하지 않습니다.

myFunction();

함수 스코프

함수는 각자의 스코프가 있음을 명심하세요 — 변수를 전역으로 선언하거나 함수에서 값을 리턴하지 않는한 함수 밖에서 함수 내부에 설정된 변수 값에 접근 할 수 없습니다. (즉, 어떠한 함수 내에서도 접근 안됨)

리턴 문 이후 코드 실행

함수 밖으로 값을 리턴할 때, JavaScript 인터프리터가 함수를 완전히 빠져나감을 명심하세요. — 리턴 문 이후에 선언된 코드는 절대로 실행되지 않습니다.

사실, 몇몇 브라우저들은 (Firefox 처럼) 리턴 문 이후에 코드가 있다면 개발자 콘솔에 에러 메세지를 줍니다. Firefox는 "unreachable code after return statement" 라고 알려줍니다.

보통 자바스크립테어서 무언가를 할당하려고 할때, 단일 등호 기호를 사용합니다. 예:

const myNumber = 0;

하지만, 이 방법은 오브젝트에선 동작하지 않습니다. — 오브젝트에선 멤버와 값 사이를 콜론으로 구분하고, 각 멤버들은 콤마로 구분합니다, 예를 들어:

const myObject = {
  name: "Chris",
  age: 38,
};

• JavaScript란 무엇인가?
• 변수는 무엇인가?
• 문자열은 무엇인가?
• 배열은 무엇인가?
• 루프는 무엇인가?
• 함수는 무엇인가?
• 이벤트는 무엇인가?
• 오브젝트는 무엇인가?
• JSON은 무엇인가?
• web API는 무엇인가?(en-US) (en-US)
• DOM은 무엇인가? (en-US)

• How do you add JavaScript to your page?
• How do you add comments to JavaScript code?

• How do you declare a variable?
• How do you initialize a variable with a value?
• How do you update a variable's value? (also see Assignment operators)
• What data types can values have in JavaScript?
• What does 'loosely typed' mean?

• What types of number do you have to deal with in web development?
• How do you do basic math in JavaScript?
• What is operator precedence, and how is it handled in JavaScript?
• How do you increment and decrement values in JavaScript?
• How do you compare values in JavaScript? (e.g. to see which one is bigger, or to see if one value is equal to another).

• How do you create a string in JavaScript?
• Do you have to use single quotes or double quotes?
• How do you escape characters in strings?
• How do you join strings together?
• Can you join strings and numbers together?
• How do you find the length of a string?
• How you find what character is at a certain position in a string?
• How do you find and extract a specific substring from a string?
• How do you change the case of a string?
• How do you replace one specific substring with another?

• How do you create an array?
• How do you access and modify the items in an array? (this includes multidimensional arrays)
• How do you find the length of an array?
• How you add and remove array items?
• How do you split a string into array items, or join array items into a string?

• What are the basic types of error?
• What are browser developer tools, and how do you access them?(en-US) (en-US)
• How do you log a value to the JavaScript console?(en-US) (en-US)
• How do you use breakpoints, and other JavaScript debugging features?(en-US) (en-US)

For more information on JavaScript debugging, see Handling common JavaScript problems(en-US) (en-US); also see Other common errors for a description of common errors.

• How do you execute different blocks of code, depending on a variable's value or other condition?
• How do you use if ...else statements?
• How do nest one decision block inside another?
• How do you use AND, OR, and NOT operators in JavaScript?
• How do you conveniently handle a large number of choices for one condition?
• How do you use a ternary operator to make a quick choice between two options based on a true or false test?

• How do you run the same bit of code over and over again?
• How do you exit a loop before the end if a certain condition is met?
• How do you skip to the next iteration of a loop if a certain condition is met?
• How do you use while and do ... while loops?
• How to iterate over the elements in an array
• How to iterate over the elements in a multidimensional array
• How to iterate over the members in an object
• How to iterate over the members of an object nested inside an array

• How do you find functions in the browser?
• What is the difference between a function and a method?
• How do you create your own functions?
• How do you run (call, or invoke) a function?
• What is an anonymous function?
• How do you specify parameters (or arguments) when invoking a function?
• What is function scope?
• What are return values, and how do you use them?

• How do you create an object?
• What is dot notation?
• What is bracket notation?
• How do you get and set the methods and properties of an object?
• What is this, in the context of an object?
• What is object-oriented programming?
• What are constructors and instances, and how do you create them?
• What different ways are there to create objects in JavaScript?

• How do you structure JSON data, and read it from JavaScript?
• How can you load a JSON file into a page?
• How do you convert a JSON object to a text string, and back again?

• What are event handlers and how do you use them?
• What are inline event handlers?
• What does the addEventListener() function do, and how do you use it?
• Which mechanism should I use to add event code to my web pages?
• What are event objects, and how do you use them?
• How do you prevent default event behaviour?
• How do events fire on nested elements? (event propagation, also related — event bubbling and capturing)
• What is event delegation, and how does it work?

• What are object prototypes?
• What is the constructor property, and how can you use it?
• How do you add methods to the constructor?
• How do you create a new constructor that inherits its members from a parent constructor?
• When should you use inheritance in JavaScript?

• How do you manipulate the DOM (e.g. adding or removing elements) using JavaScript? (en-US)